you're spot on there Astrid - it will be classed a a part of your income - if you haven't estimated it, you'll get a nasty little debt from FAO at the end of the year - what i would suggest, if it's a "maybe it will sell this year, maybe it won't" situation - keep a record of what you're receiving - when the land sells, put that much aside so that, as soon as you get that little letter saying "you owe x amount" you can pay it back

it happens frequently as property sales are so variable - so if you're prepared for it you'll be better set than most!