Reversing service entry sheet for service po

Hi Expert,

For service PO (Item cat D), if invoice posted and payment run completed and money encashed , can we reverse the entry sheet?

Here is my problem:

Service PO is for qty 100.

Entry sheet entered

SES 1     20

SES 2     30

SES 3     50

Invoice and payment run completed for everything, fully paid.

INV1     20

INV2     30

INV3     50

Now, vendor gave credit for 15 due to some business reasons.

We entered CREDIT MEMO on SES1 for 15,

PO History shows like this now

GR Total          100         

Invoice total       85

SES Total          100

Question 1: Since I have credit of qty 15, can I entered service entry sheet for qty of 15.

Question2: Can I reverse the SES 1 since the invoice if fully paid and encashed?

Question 3: When I get credit from vendor for service PO like this scenario, should I enter this as Credit Memo or Subsequent Credit?

Ay advise will be highly appreciated.

Thanks.