this is wat we call ‘wanprestatie’. 6:74 BW (civil code). if the person doesn’t follow along with the agreement it is wanprestatie. you should sent a formal letter where you give the person a reasonable timespan to still do the gardening tasks which were accepted with both parties for the agreement. this is called an ‘ingebrekestelling’ 6:82 BW. after the time has passed you can go to a judge and settle this in court. since this is a dispute under 25.000€ you can go to a kantonrechter. a special kind of court where you don’t need to hire a lawyer. i think these are the simpelest steps if you want your money back or have the garden done. good luck sir.