Is it ensure, insure, or assure?
between the early 16th and the mid-18th century, an agreement between the Spanish crown and a private person or another sovereign power by which the latter was granted a monopoly in supplying African slaves for the Spanish colonies in the Americas. The contractor (asentista) agreed to pay a certain amount of money to the crown for the monopoly and to deliver a stipulated number of male and female slaves for sale in the American markets. The first such contractor was a Genoese company that in 1517 agreed to supply 1,000 slaves over an eight-year period. In 1528 an agreement was reached with a German firm to supply 4,000 slaves. For its monopoly the firm paid 20,000 ducats annually to the crown. Each slave was sold at a price not exceeding 45 ducats.