Newer, factory marked 870 Police guns in very good used condition sell for around $400. Sometimes more, sometimes less, take a look at Gunbroker.com and search "Remington 870 Police" to see what's being bid on right now. From time to time batches of agency trade-in 870s show up on the market and they often do sell for $225-300 apiece.
Paying 'new gun' price for used guns is not something I make a habit of doing. In your case, it might well be a case of 'take this gun in exchange for the debt, or wait for an uncertain future to arrive to get full repayment.' That has more bearing on this case than the used or new price of an 870P IMHO. Depending on this person's reputation and ability to pay off the amount owed, I might well take the deal and call it even.
Even if it did mean taking a bit of a haircut on the deal. A percentage of something is better than all of nothing, IMHO.