What is the ruling on the seller stipulating that the down payment be forfeited if the buyer does not pay the remaining amount by the agreed date, and that the buyer must pay the full price of the property without counting the down payment?

It is not permissible to stipulate a penalty clause for the buyer's delay in paying the remaining price, as this is explicit usury (riba). Nor is it permissible if the buyer wishes to withdraw from the purchase deal, if you will receive the price of the shop, for there is no harm that would necessitate an additional sum. You can oblige him to bear the litigation expenses if he delays. It is advisable to stip stipulate a guarantor or a mortgage, or to not return the earnest money if he withdraws from the purchase after the contract is finalized.