Aligning multiline equation in align

Do you mean this?

begin{align}

    max quad & b_s 
    text{s.t.} quad  Af & = b 
    quad  b_d & = - b_s 
    quad  b_i & = 0 quad forall i neq s, t 
    quad  0 leq & f_{ij} leq infty

end{align}

Probably you looking for

documentclass{article}
usepackage{amsmath}

begin{document}
begin{align}
    max quad b_s 
    text{s.t.} quad Af    & = b 
                      b_d   & = - b_s 
                      b_i   & = 0, forall i neq s, t 
                      0     & leq f_{ij} leq infty
end{align}
end{document}

There is also a specific package optidef, with the option maxi!

documentclass[a4paper,12pt]{article}
usepackage{amsmath,amssymb}
usepackage{optidef}

begin{document}
begin{maxi!}|s|[2]<b>
{}{b_s}{}
{}
addConstraint{Af}{=b}
addConstraint{b_d}{=-b_s}
addConstraint{b_i}{=0, quad forall i neq s, t}
addConstraint{}{0 leq f_{ij} leq infty}
end{maxi!}
end{document}