שׁבר shbr "break". E.g. Am 1:5 where it refers to the city's "gate-bars" (see e.g.. Paul p.51).
שֵׁבֶר sheber "breaking". In Am 6:6 speaking of the decay and spoiling of the Northern kingdom the "breaking (or 'ruin') of Joseph".
שׁבר shbr "to buy [qal], sell [hiphil] grain". The hiphil appears in Amos, in successive verses (8:5-6), but speaking of very different objects being sold. In v.5 the merchants, on the Sabbath, want to sell "corn"; in v.6 they speak of selling "the husks of wheat"!
שֶׁבֶר sheber "corn" or more generally "grain". In Am 8:5 in parallel with "wheat". (One could therefore render "grain" making the second line more precise, but I have chosen "corn" suggesting that the precise product is not important.)