Would I be sinful if I mediated the marriage of a man and a woman, knowing that they will live in an apartment which the questioner believes its purchase through the bank's installment system involves a suspicion of usury?

Striving to marry off Muslims is a righteous deed, falling under cooperation in piety and righteousness. This is because marriage is the Sunnah (definition) of the prophets and messengers, and through it, half of one's religion is perfected. The questioner's effort in marrying off Muslims is a good deed for which he will be rewarded. It is not affected by the wife residing in a property that the husband bought through unlawful means, and there is no sin upon the one who facilitated the marriage in this regard.