What is the ruling on the the mosque's committee selling a rug that a man donated to the mosque, and buying another rug with its price?

The endowed mosque carpets should not be sold, because the basic principle is that it is impermissible to sell an endowment (waqf). It would have been more appropriate to transfer them to another mosque that needs them. However, since they have already done so, some scholars have permitted selling an endowment and replacing it with something better for the common good. Nevertheless, one should not be lenient in this matter.