OK, you didn't show any test data, so I used my own. This should get you started:
the new record-type
both/any (if present) will have same low-order key
and here's the value to SUM
for all others
low-order key is unique per record
dummy value of zero for amount for non-150s
000 (to make non-unique) or sequence number (to make unique)
non-zero for 150s only
your fixed-length record length
to identify the start of a GROUP as SyncSort does not have KEYBEGIN
sequence number within group
You'll need to change positions, lengths, types as necessary.
The idea is, before the SORT/SUM, change the input types as necessary, and have a new sort-key which is non-unique (I used zero) for the 150s, unique for all others. Extend the record to hold the amount for summing for 150s, zero for all others. Then SORT on all your keys (I ignored one) plus the new unique/non-unique key. SUM on the extended amount.
Use OUTREC to establish sequence number and to put the SUMmed amount in the correct place for the single 150 record (if present).