What was a land payment that the Spanish government gave to conquistadors.
The word that you are looking for is encomienda. The system of encomiendas fits pretty well with the definition that you have given here.
In the encomienda system, the Spanish monarchy rewarded the conquistadors for conquering new lands for Spain. What they did was to give the conquistadors the rights to the labor of a given number of Indians. Technically, the encomenderos (the men who were given the encomienda) did not actually own the land. The land technically still belonged to the Indians. But the encomenderos were given the right to force the Indians to work for them or to pay them tribute in others forms.
So this was not technically a grant of land, but the effect (while the system lasted) was much the same as if it were an outright grant of land.