I figured out something that appears to be working for my purpose... Here's what I came up with:
StringVar CommentSpacer;
CommentSpacer := CHR(13) + CHR(10);
StringVar PayMsg;
StringVar AmtFromPayMsg;
local numbervar start;
local numbervar comma;
local numbervar amt_length;
PayMsg := {APY2021.PYMNT_MESSAGE.B};
start := instr({APY2021.PYMNT_MESSAGE.B}, ":")+1;
comma := instr({APY2021.PYMNT_MESSAGE.B}, ",");
amt_length := (comma - start);
IF PayMsg <> ""
THEN AmtFromPayMsg := mid(PayMsg, start, amt_length)
ELSE AmtFromPayMsg := "";
// do NOT want to see the Prepayment, Invoice from system
// make it more of a standard Details a line
if {APY2021.PYMNT_MESSAGE.B} LIKE 'Prepayment*'
AND {APY2021.VOUCHER_STYLE} = 'REG'
and {APY2021.PAID_AMT} = 0
Then
TRIM({APY2021.DESCR254_MIXED.K}) + CommentSpacer
+ 'PREPAID: Invc ' + {APY2021.INVOICE_ID}
+ ' '
+ ' '
+ '$' + AmtFromPayMsg
+ CommentSpacer + CommentSpacer
else if {APY2021.VOUCHER_STYLE} = 'REG'
and {APY2021.PAID_AMT} = 0
Then {APY2021.DESCR254_MIXED.K} ;
IP Logged