Hi Hari,
one option is to set file modification check in the sender file adapter (select Advanced mode)
the sender channel will wait if the file is being modified.
for more information check the below link
https://help.sap.com/saphelp_nwpi711/helpdata/en/44/682bcd7f2a6d12e10000000a1553f6/frameset.htm
another option is to modify your program that writes the file, to write the file as .tmp and once the transfer is complete, rename with the actual filename. sender file adapter will only pick once the write is complete and file is renamed to actual filename