Short answer. Not for any shortfall. Article 85 of the Family Code validates a donation by reason of marriage even where the property is mortgaged, and if it is later foreclosed and sold for less than the debt it secured, you as the donee are not liable for the deficiency. If it sells for more, the excess belongs to you.
What the law says
Donations by reason of marriage of property subject to encumbrances shall be valid. In case of foreclosure of the encumbrance and the property is sold for less than the total amount of the obligation secured, the donee shall not be liable for the deficiency.
Family Code, Article 85 — Donations of Encumbered Property. Read the full provision →
The gift itself is not invalid because it is mortgaged
Article 85 removes any doubt about whether a mortgaged property can validly be given as a wedding gift: donations by reason of marriage of property subject to encumbrances shall be valid. The existence of a mortgage does not stop your fiance from making the donation, and it does not make the gift to you ineffective. The encumbrance simply comes along with the property as given.
You are protected from a deficiency after foreclosure
The article then addresses exactly the worry behind this question: in case of foreclosure of the encumbrance and the property is sold for less than the total amount of the obligation secured, the donee shall not be liable for the deficiency. If the mortgage is later foreclosed and the sale does not cover the full loan balance, that shortfall is not your responsibility as the donee. The lender's remedy for the deficiency does not extend to you simply because you received the property as a gift.
The flip side: you keep any excess
Article 85 also covers the opposite outcome: if the property is sold for more than the total amount of said obligation, the donee shall be entitled to the excess. So the article does not just shield you from a downside — it also gives you the benefit if the foreclosure sale brings in more than what was owed on the loan. Both directions of that outcome belong to you as the donee, not to the donor or the lender.
What this article does not address
Article 85 speaks specifically to what happens if the encumbrance is foreclosed. It does not itself address who is expected to keep making loan payments before any foreclosure happens, or what arrangement, if any, exists between you and your fiance regarding the debt while the marriage relationship and the mortgage are both still in place. Those questions depend on facts and arrangements this article does not cover.