we are praying taraweeh in Ramadan. Can the imam of the masjid collect donations in the middle of the salat? or he should wait till the end of salat.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

We do not know of anything that prohibits someone from raising funds for charitable activities during the break between Taraaweeh prayers. Performing Taraaweeh prayers consecutively is not among the conditions for its validity as far as we know. So, there is nothing to support the prohibition of collecting donations during the break between Taraaweeh prayers.

However, it would be better if the Imaam waits until after performing the prayers so as not to delay people and keep them waiting. Perhaps some of the worshippers are sick, (physically) weak, or need to carry out an errand and if they have to wait for a long time before leaving the mosque, this may be inconvenient and difficult for them.

Allaah Knows best.