How to sum up with conditions (complicated)

Hi there,

I have a quit complicated question about programming with Exel. I tried to Google it and some formulas myself but it wasn’t working - maybe somebody has a solution for me. 

I have just added a picture of the sheet to visualize the problem. 
I want to fill the empty gaps in column C with the summed amounts from the

transaction from column B with the same transaction number from column A. Unfortunately these gaps are on different position. It’s a very long sheets why it’s impossible to to it manually. 
Does anyone has an idea?

P. S. If it’s easier I could also go with the results from column C being in the column of B but as an negative amount. (And deleting column C)

thank you so much!