In our small city there are very few Shias and no centre or Hussainiya. In this situation is it allowed to donate for the building of a Sunni mosque?
If you are sure that the Masjid will remain under the management of reasonable persons and will not be influenced by extreme Wahabi groups, then you can help them.
It is better to help poor Mo'mineen in poor areas or poor countries where they don't have a Masjid.
Wassalam.