At this point it appears President Trump had nothing to do with the conversation between a House member and Wikileaks' leader Julian Assange. But even if the President were involved, there is nothing wrong with law enforcement officials offering immunity for evidence. It happens all the time in prosecutor's offices all across America.
If someone did have evidence regarding the actual party responsible for the DNC email hack or theft wouldn't it be worthwhile to promise them either a pardon or immunity from prosecution in order to get the evidence?
+100 - Operation Varsity Blues (college admissions bribery scandal) was a recent example of a perp trading a deal for information (see Morrie Tobin, tipster) .