Short answer. Split it. Payments on the principal collected during the marriage remain your exclusive property, because the credit was yours before the wedding. The interest that falls due during the marriage belongs to the conjugal partnership. The same cheque may therefore contain both kinds of money.
What the law says
the sums which may be collected during the marriage in partial payments or by installments on the principal shall be the exclusive property of the spouse. However, interests falling due during the marriage on the principal shall belong to the conjugal partnership.
Family Code, Article 119 — Credit Payable in Installments. Read the full provision →
Collecting is not earning
The reason principal stays exclusive is that nothing new is being acquired when it is repaid. The credit existed before the wedding and already belonged to you; each instalment merely converts a receivable you owned into cash you own. Timing does not change ownership, so the fact that the money arrives during the marriage is beside the point. Interest is different in kind. It is a fruit produced by the capital over time, and under the conjugal partnership the fruits of each spouse's separate property accrue to the partnership — which is why the article splits a single payment down the middle.
Only interest that falls due during the marriage
The qualifier matters. What belongs to the partnership is interests falling due during the marriage, not all interest ever earned on the loan. Interest that accrued before the wedding but was collected afterwards was already yours when it fell due, and it does not change character by being paid late. So the dividing date is the celebration of the marriage, applied to when each amount became due rather than when the borrower actually handed it over. A borrower who was two years in arrears at the wedding can produce a payment that is exclusive property in its entirety.
This is a conjugal partnership rule
The article sits within the conjugal partnership of gains, which is not the default regime for marriages under the Family Code — the default is absolute community, where the analysis is different because most property owned before the marriage is pooled from the start. So before applying this rule, be sure of which regime governs you, which turns on whether you executed a valid marriage settlement choosing conjugal partnership and, for older marriages, on the law in force when you married. Getting the regime wrong makes every subsequent calculation wrong.
Make the borrower's payments itemised
Practically, everything depends on a document nobody bothers with until there is a dispute: an amortisation record that separates principal from interest, payment by payment, with dates. If the loan instrument states a rate and a schedule, that is the backbone. If payments have been lump sums with no allocation, expect an argument about how each was applied. Keep the original promissory note or deed showing the loan predates the marriage, and record each receipt as principal or interest at the time you take it, not years later from memory.