Spending money donated to build a Masjid in a certain country on nothing else

Q: We are a group of Muslims who are studying our PhD in Poland. We intended to build a Jami` (a large mosque where Friday Prayers are held) in one of the cities here. By virtue of Allah (Exalted be He) we collected a sum of money for that purpose from different sources, some of which were Muslims living in Poland and some others were Muslims living in Germany. However, no actual procedures have started for building the Jami`. We have been waiting for the approval of the concerned authority so that we may start building on a piece of land that the owner agreed to sell to us. Thus, the procedures of buying the land and starting the construction work will commence soon In sha’a-Allah (if Allah wills). We have come across some questions that we hope Your Eminence could give appropriate answers for according to the purified Shari`ah (Islamic law) so that the benefit of Islam and Muslims is achieved. These questions are:1- A Muslim brother collected some donations totaling ten thousand dollars from Muslims living in Poland. People waited for a long time but working on the Jami` did not start. Consequently, the sum mentioned above was needed to buy a piece of land in Egypt to build another Jami` on it. (Part No. 16; Page No. 50) Accordingly, the concerned brother requested to recover the sum that he collected from Muslims who donated with the intention of building a Jami` in this city in Poland. Some of the donators have already traveled after finishing their studies. Should we return the sum to the brother in order to achieve the other objective or not? Please guide us! May Allah reward you with the best for helping us.2- The brother who keeps the sum of ten thousand dollars that was donated for building the Jami` may leave the country after a short period of time when he finishes his studies. No other person agrees to assume the responsibility and keep the money. How can we overcome this problem? Is it permissible for us to put this money in the bank? It may be worthy to mention that banks give compulsory interest. Please clarify.3- If the process of building the concerned Jami` fails, may Allah forbid; should we repay the different sums to all those to whom they are due in case we can find them and we know the value of the sums that they donated, or should these sums be spent on the different spheres of Da`wah (calling to Islam) in this country? Please clarify.May Allah grant you success and protect you and may He reward you with the best for benefiting us.