Say you have a capacity constraint on an edge from u to v of -3, what does this mean?
Well, by definition, it means that you can't push more than -3 units of flow from u to v; meaning the flow from u to v could be, for example, -5, -4, or -3. If we posit, as is common, that flow of -x from u to v is the same as flow of x from v to u then we see these example flows of -5 or -4 or -3 would translate to flows of 5 or 4 or 3 from v to u and, further, that the flow from v to u could not be less than 3.
Thus we see a maximum capacity of -x from u to v is equivalent to minimum capacity constraint of x from v to u and the problem of handling negative capacity constraints in a max flow computation therefore reduces to the problem of handling both maximum and minimum positive-only capacities.
One can handle minimum and maximum capacities by first finding a feasible flow on the graph, a flow that satisfies flow rules and capacity constraints but that is not necessarily a maximum flow -- this can be done by finding a "circulation" on a specially constructed graph derived from the original graph, and then turning the feasible flow into a max flow by solving a max flow problem. This technique is discussed in detail at the following link: max flow slides