the custentity_paid_against_credit is an inline html field - but I get the following error: INVALID_FLD_VALUE The field custentity_paid_against_credit contained more than the maximum number ( 15 ) of characters allowed. Is it really 15 character limit or is something else going on the URL variable?

hmm, looks like custom scripting for such field is not supported - thanks for the link

set the default value serverside

do you mean to generate the URL for the inline field on serverside?

yes