\starttext
\startMPcode
u:=1cm ;
path p, q ;
p=fullsquare scaled u ;
q=fullsquare scaled u shifted (4u,-2u) ;
draw p ;
draw q ;
drawarrow center p--center q ;
label(btex $5$ etex,center p) ;
label(btex $-2 $ etex,center q) ;
\stopMPcode
\stoptext

How about something like below. I'm not sure exactly how ncline works in PStricks, but this macro takes 4 arguments, namely a point, its corresponding label, a second point, and its corresponding label.

\starttext
\startMPcode

vardef ncline (expr A,h,B,k) =
    save p,q;
    path p,q;
    label(h,A);
    p:=fullsquare scaled u shifted A;
    draw p;
    label(k,B);
    q:=fullsquare scaled u shifted B;
    draw q;
    drawarrow (A--B) cutbefore p cutafter q;
enddef;

u:=1cm ;
ncline(origin,btex $5$ etex,(4*u,-2*u),btex $-2$ etex);

\stopMPcode
\stoptext

Troy Henderson