Commission charges

Hi all, New to this, not an accountant, and just having a play before I have to do this for real!! I am a self employed handyman. Turnover is less than 30K. I deal with letting agents. My invoices are made up for full amount, including agents commission (their rules, so cant change this). They pay my invoice, less commission.

Example, my invoice is 170, I get paid 129,20, 40.80 commission. I can link the 129.20 to my invoice from the bank import. How do I record the commission, and link it to the invoice, so it reconciles the whole invoice? Simple terms please : )