Like the first comment said, how do they trust each other?
How did the buyer make sure you are not sending him a porn movie? I think the engineer knows all about that, but meeting is inevitable.
They don't. The Russian agent places no value on the money. If it doesn't result in any useful information then it is a minor business loss. The American agent has accepted money from the Russian. He is already on the hook for espionage charges. He only continues the relationship to keep the Russian happy, to keep the money coming but also to keep the Russian from informing the FBI. The business deal becomes a continuing blackmail operation. Nobody trusts anyone.
Historically, the USSR was really cheap about paying spies in the US. Maybe the GRU is paying better.