You can and should talk to the seller and see if he or she will extend you more time to pay, which is his or her right. However, doing so is voluntary on the seller's part; if the seller chooses to not do so, he or she can enforce the plain terms of the contract against you, which means that you will be in default or breach. Presumably (in the typical case, though you do need to check the language of your specific contract) this means you'll lose the land, not get any money paid already back, and the seller could also sue you for the remaining money due under the contract.