There probably are better ways but here goes.
Let each transaction be denoted $(i,A_i)$ and let them arrive in a stream.
Initialize a list $L:=$ which will be sorted on the second component.
For the first $K$ transactions insert $(i,A_i)$ into the proper position in $L.$ Let $L$ denote the smallest entry in $L.$
Since the maximum amount seen so far is ...