Why not have caps based on nation strength?
sqrt(Strength) * 100,000 = money cap
sqrt(Strength) = tech cap
My strength is at a measly 30k with 1.8k tech. When dealing with a bigger nation, I would be able to send $17 million and send 173 tech. When dealing with a smaller one, I should only be able to deal their limit and not mine. The system should work to take the minimum of the two dealing nations.