Items and pickups for sale in Shops can be bought when Isaac does not have enough money to afford them. Items and pickups bought this way will result in the remaining amount of money that was owed accumulating as "debt", shown as a negative number next to Isaac's coin counter on the HUD, which gives Isaac a multiplicative damage down for every point of debt he has.
Gaining money while Isaac has any amount of debt will subtract that amount from it, instead of adding to Isaac's held coins, until there is no debt left.
The maximum amount of debt Isaac can have is 99, resulting in a 0.1× damage multiplier. Isaac will not be allowed to buy anything that would result in the debt going above this cap.
Notes
The multiplicative damage down is calculated with the linear interpolation formula: (1 − (Debt ÷ 99)) + 0.1 × (Debt ÷ 99)
Removing Dad's Wallet will not remove any debt Isaac has, which will remain until it is fully paid off.
Dad's Wallet cannot be used to pay Machines and Beggars when Isaac has no held coins.
Dad's Wallet will not affect items for sale that can't be paid with money, such as Devil Deals.
Credit Card upon being picked up.
Shops can be bought when Isaac does not have enough money to afford them. Items and pickups bought this way will result in the remaining amount of money that was owed accumulating as "debt", shown as a negative number next to Isaac's coin counter on the HUD, which gives Isaac a multiplicative damage down for every point of debt he has.
Boom Flies to become 
Golden Boom Flies.
Deep Pockets: Debt will still be capped at 99 coins and won't increase further.
Keeper / 
A Pound of Flesh / 
Keeper's Bargain: Dad's Wallet can be used to buy Devil Deal and Black Market items for sale that now require money.
