Hi Jennifer,
From what I can see in your screen-shot is that the 'Clearing Amount' under the items tab belongs to that open document 6700308829 and this is fixed for the document cannot be changed.
To achieve partial clearing, Payment amount 80 and the open document amount 90, depending on the Clearing Rules, if you just run the Payment Run or do Account Maintenance (FP06/FPMA), you should achieve this partial clearing.
Spro path for Clearing Steps:
spro -> Financial Accounting (New) -> Contract Accounts Receivable and Payable -> Basic Functions -> Open Item Management -> Clearing Control -> Define Clearing Variants.
Hope this helps.
Thanks,
Srini