Reference for Megillah 56:5

If at the time of building the synagogue, they made a condition to be able to use it [for mundane purposes], it is permitted to use it after it is destroyed, but while it is in use, the condition is ineffective. Even when it is destroyed, to use it for a disgraceful purpose, like planting or financial calculations for the people, the condition is ineffective. What are we dealing with? With synagogues outside of Israel; but with synagogues in Israel, no condition is effective.