Is it obligatory to compensate for the deficit that occurred in the amount allocated for building a mosque when it was converted to another currency, which led to a 25% decrease in its value?

Merely pledging to perform acts of devotion does not make them obligatory unless they are in the form of a commitment or a covenant with Allah. If what you pledged was a vow (nadhr), then you are obliged to fulfill it. If it was an oath (yameen), you have the choice between fulfilling it or offering expiation (kaffarah).

If you did not obligate yourself with what you pledged, then there is no blame upon you for transferring the amount to another mosque, and you are not required to compensate for any deficit resulting from currency exchange. However, if the pledge was a vow (nadhr), then you are obliged to fulfill it, and it is permissible to transfer the remaining money to another mosque if it is impossible to benefit from it in the first mosque. In this case, you are obliged to compensate for any deficit in the vowed money from your own personal funds, not from the vowed money itself.