It is confusingly written, but what it means by repayment in the above paragraph is not "until you get your 1200 guilders back in these 75-guilder installments", its meaning is clarified by the next paragraph,
> On such conditions that I or my successors [unreadable] of the aforesaid Leckendijck may, at any time when it pleases us, extinguish, repay, and buy back the said annuity in full at once and not in parts or fractions with the sum of one thousand two hundred Carolus guilders
which explains how the board can get itself out of this obligation: by paying the original principal of 1200 guilders back.