The issue is where is the physical $30 held at each stage. Look at it in each discrete stage of the money flow and assume for simplicity that the $30 is all there is between all the parties.
A. Before paying: Man_A has $10; Man_B has $10; Man_C has $10; Desk_Clark has $0; Bell_Boy has $0
This gives a total of $30 flowing between the parties.
B. After paying $10 each: Man_A has $0; Man_B has $0; Man_C has $0; Desk_Clerk has $30; Bell_Boy has $0
This again gives a total of $30 flowing between the parties.
C. After giving the $5 to the Bell Boy: Man_A has $0; Man_B has $0; Man_C has $0; Desk_Clerk has $25; Bell_Boy has $5
This again gives a total of $30 flowing between the parties.
D. After the Bell Boy gives $1 refund to each Man and pocketing a $2 "tip": Man_A has $1; Man_B has $1; Man_C has $1; Desk_Clerk has $25; Bell_Boy has $2
This again gives a total of $30 flowing between the parties.
So the paradox is resolved by tracing the physical money not by combining what the three men eventually paid with tip taken by the bell boy.
