Dedicating a Masjid as Waqf built by a usurious loan

Q: We inquired about the Masjid (mosque) where we offer Salah (Prayer). The building where we offer Salah is owned by a Muslim brother who lives here. The Muslim community used to rent this building from that man in return for a monthly rent. Then, this man decided to donate the building to be a permanent Masjid for the Muslims here, and he refused to take money. His intention was good. However, he trades in foodstuff and real estate. He also acknowledged that about one third of (Part No. 16; Page No. 33) his trade is in alcoholic drinks.In addition, he told us that he had bought the building where we offer Salah through a loan from a usurious bank. He is still repaying the loan, in addition to the interests. We asked Your Eminence over the telephone whether we should accept this man's donation, given that a part of his money is acquired through trading in alcoholic drinks and Riba (usury). Your answer was as follows: you should accept the donation and stop asking the man about the source of this money. We should also make him write an acknowledgement that he has donated the building to be used as a Masjid.We told him your Fatwa (legal opinion issued by a qualified Muslim scholar), and we asked him to write an acknowledgement that he has donated the Masjid to a reputable Islamic entity, such as the office of the Muslim World League in the USA. However, he refused to write an acknowledgement that he has donated the Masjid; he only pledged that this building will be a Masjid forever. He was afraid that anyone might use this building for another purpose later, or that the Masjid will be used by some group or entity to publicize themselves. We tried hard (Part No. 16; Page No. 34) to convince him, but it was of no use.As for the current situation, we are divided into two parties: the first party is of the view that there is no harm in offering Salah in this Masjid, even if the building owner does not write the acknowledgement; and the other party is of the view that this acknowledgement should be written; otherwise it is impermissible to offer Salah there. Please, Your Eminence, advise us concerning what we should do. Is it permissible to offer Salah in that building? We would like you to also give us your response in writing so that we can distribute it in the Masjid. Many thanks in advance. May Allah guide us all to what is right. May Allah reward you.