In Re the Marriage of: Patricia M. Welinski, petitioner, Respondent, vs. Benet D. Welinski, Appellant.

may not be cited except as provided by

Minn. Stat. § 480 A. 08, subd. 3 (1998).

 STATE OF MINNESOTA

 IN COURT OF APPEALS

 C4-98-1302

In Re the Marriage of:

Patricia M. Welinski, petitioner,

Respondent,

vs.

Benet D. Welinski,

Appellant.

 Filed January 26, 1999

 Remanded

 Peterson, Judge

Hennepin County District Court

File No. 192171

Betsy S. Anding, 300 Southdale Place, 3400 West 66th Street, Edina, MN 55435 (for respondent)

Alan J. Albrecht, Albrecht & Associates, Ltd., Boulevard Plaza, 7066 Brooklyn Boulevard, Brooklyn Center, MN 55429 (for appellant)

Considered and decided by Peterson, Presiding Judge, Schumacher, Judge, and Harten, Judge.

 U N P U B L I S H E D O P I N I O N

 PETERSON, Judge

Appellant-father Benet Welinski argues that the district court should have credited him for what he alleges are prior overpayments of support he made to respondent-mother Patricia Welinski. Because this is a discretionary issue, and the district court apparently ruled as a matter of law, we remand.

 FACTS

The 1995 judgment dissolving the parties' marriage awarded the parties joint physical custody of their two children, offset their support obligations, and set a net obligation for father. Later, one child started living with each parent, but the parties' support obligations were not adjusted to reflect this fact. In May 1998, father moved to modify support and sought a credit against future support payments for his prior "overpayments" of support. The district court denied the requested credit. Father appeals.

 D E C I S I O N

Child support may be modified if the moving party shows a substantial change in circumstances rendering the existing support award unreasonable and unfair. Minn. Stat. § 518.64, subd. 2(a) (Supp. 1997). Whether to modify support is discretionary with the district court, and we will not alter the district court's decision absent an abuse of its broad discretion. Moylan v. Moylan, 384 N.W.2d 859, 864 (Minn. 1986).

1. Because the support father previously paid was the amount required by the judgment, the only way that the payments father made can be characterized as overpayments is if father's support obligation is reduced retroactively. Minn. Stat. § 518.64, subd. 2(d) (Supp. 1997), limits the retroactive modification of support. Father argues that the limitations set forth in Minn. Stat. § 518.64, subd. 2(d), apply only when the moving party seeks to avoid paying arrears and, because he is not trying to avoid paying arrears, he concluded the statute does not apply to prohibit retroactive modification of his support obligation. We disagree.

By its terms, Minn. Stat. § 518.64, subd. 2(d), applies to all attempts to modify support retroactively; its applicability is not limited to circumstances where the moving party seeks to avoid paying arrears. To conclude that the statute does not apply to father's motion seeking credit for past overpayments of support, we would have to read into the statute a limitation that is not there. We decline to do so. See Minn. Stat. §§ 645.16 (1996) ("[w]hen the words of a law in their application to an existing situation are clear and free from all ambiguity, the letter of the law shall not be disregarded under the pretext of pursuing the spirit") (emphasis added), 645.44, subd. 16 (1996) ("`[s]hall' is mandatory"); see also Martinco v. Hastings, 265 Minn. 490, 497, 122 N.W.2d 631, 638 (1963) ("[i]f there is to be a change in the statute, it must come from the legislature, for the courts cannot supply that which the legislature purposely omits or inadvertently overlooks").

2. Relief similar to the type father seeks was granted in Karypis v. Karypis, 458 N.W.2d 129 (Minn. App. 1990), review denied (Minn. Sept. 14, 1990). Father claims the district court should have granted him that relief.[1] Karypis is based on a district court's discretionary exercise of its "`equitable jurisdiction" under which "relief may be awarded as the facts in each particular case and the ends of justice may require.'" Id. at 131 (quoting Johnston v. Johnston, 280 Minn. 81, 86, 158 N.W.2d 249, 254 (1968)). Much of Karypis was later codified in a statute that gave the district court discretion to decide whether a support obligor has satisfied a support obligation by letting a child live with him. See 1991 Minn. Laws ch. 266, § 3 (codified at Minn. Stat. § 518.57, subd. 3 (1996)) (stating court "may" rule support obligation "satisfied" by obligor's providing home, care, and support for child living with obligor if child is integrated into obligor's home with support recipient's consent) (emphasis added); Minn. Stat. § 645.44, subd. 15 (1996) ("`[m]ay' is permissive").

When father sought credit for the previously paid support, he cited the district court to Karypis and Minn. Stat. § 518.57, subd. 3. The order denying father's motion, however, mentions neither Karypis nor the statute and states: "A child support modification can only be retroactive to the date of a pending motion for modification." Thus, apparently, the district court ruled as a matter of law on what caselaw and the statute say is a discretionary issue. We remand for the district court to exercise its discretion. See In re Welfare of M.F., 473 N.W.2d 367, 370 (Minn. App. 1991) (where district court erroneously ruled on a discretionary issue as a matter of law, case remanded for exercise of district court's discretion).

3. We express no opinion on how to resolve this case on remand. Also, on remand, the district court shall have discretion regarding whether to reopen the record.

Remanded.

[1] Father also cites an unpublished opinion to support his argument. Unpublished opinions are not precedential and are of limited value in deciding an appeal. See Minn. Stat. § 480 A. 08, subd. 3(c) (1996) ("[u]npublished opinions of the court of appeals are not precedential") (emphasis added); Dynamic Air, Inc. v. Bloch, 502 N.W.2d 796, 800-01 (Minn. App. 1993) (stating dangers of miscitation and unfairness associated with use of unpublished opinions and that while persuasive, "[t]he legislature has unequivocally provided that unpublished opinions are not precedential").