If the receiving system doesn’t care that the number is sequential, just needs to be different, make MSH-10 equal to 12345 for the new transaction and be done with it.
Since you are only spinning off 1 message and following the spin off, a new message with a new MSH-10 will come thru so you won’t have to worry about counters or duplicates.
To add my 2 cents, I agree with the rest of those who think the engine should not create charges.
Hope this helps,
-mh
