From another many year old news posting: if you picked up a stack
of potions of oil in a shop and then applied them, one potion was split
off and started burning but you were forced to pay for all of them.
Split the to-be-lit one off first so that the remainder of the stack
stays as ordinary unpaid shop goods.
This also fixes an old bug with bill_dummy_object sometimes charging
a different price than the player got quoted when an object was picked up.
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