Lifting relative homotopies between maps $X\to Y/B$ when $B$ is contractible Let $(X,A)$ be a CW-pair, $Y$ a CW-complex, and $f,g:X\to Y$ homotopic maps such that $f_{|A}=g_{|A}$. Even though $f$ and $g$ are homotopic, they do not have to be homotopic relative $A$. (Obstruction theory tells us how to deal with this issue.)
Let us further assume that $B\subseteq Y$ is a contractible subcomplex and that the compositions $X\stackrel{f}{\longrightarrow}Y\stackrel{pr}{\longrightarrow} Y/B$ and $X\stackrel{g}{\longrightarrow}Y\stackrel{pr}{\longrightarrow} Y/B$ are homotopic relative $A$.
I haven't thought about this for very long, but shouldn't this already imply that the original maps $f$ and $g$ are homotopic relative $A$?
(If this is true, then we can probably drop the assumption of $f$ and $g$ being (freely) homotopic.)
Sebastian
 A: Sure. The fact that $A$ is a subcomplex of $X$ implies that the restriction maps $$Map(X,Y)\to Map(A,Y)$$ and $$Map(X,Y/B)\to Map(A,Y/B)$$ are Serre fibrations. The fact that $B$ is a subcomplex of $Y$ and contractible implies that the projection $Y\to Y/B$ is a homotopy equivalence, which in turn implies that the resulting maps $$Map(X,Y)\to Map(X,Y/B)$$ and $$Map(A,Y)\to Map(A,Y/B)$$ are homotopy equivalences and in particular weak equivalences. It follows that the associated map of fibers $$Map(X,Y \ rel A)\to Map(X,Y/B\ rel A)$$ is also a weak equivalence, in particular injective on $\pi_0$.
A: This example is wrong (sorry!):
Try $(X,A) =   (D^n, S^{n-1})$,  $Y = D^n$,   $f_A = \mathrm{in}_{S^{n-1}}$, and $B = Y$.
Then we have plenty of nonequivalent (rel. $A$) maps $f, g: X\to Y$, and they 
all become equal in $Y/B = *$.
Since $D^n$ is convex, any two maps $X\to D^n$ are homotopic by straight-line homotopies;  and if the maps agree on some subset of $X$, the homotopy will be constant on that subset.
A: Alright, here is an idea for when $A$ is a skeleton; let's say $A=X^{n-1}\subseteq X$ is the $(n-1)$-skeleton. For now assume that $X$ is $n$-dimensional, the statement for arbitrary $X$ should follow by induction.
From my understanding, $f$ and $g$ are homotopic relative $A$ if and only if their difference cochain $d(f,g)\in C^n(X;\pi_n(Y))$ vanishes (cf. Mosher and Tangora, Theorem 4). So let's assume that $d(f,g)\neq 0$, that is $f$ and $g$ are not homotopic relative $A$, and proceed with all the assumptions made above.
Since $B$ is assumed to be a contractible subcomplex of $Y$, the projection $pr:Y\to Y/B$ is a homotopy equivalence and induces an isomorphism $\pi_n(pr):\pi_n(Y)\to\pi_n(Y/B)$. Furthermore, we also get an induced map $p_\ast:C^n(X;\pi_n(Y))\to C^n(X;\pi_n(Y/B)),\ \phi\mapsto\pi_n(pr)\circ\phi$, which is injective by the injectivity of $\pi_n(pr)$. Thus $p_\ast(d(f,g))\neq 0$, where by the functoriality of $\pi_n(-)$ we (should) have $p_\ast(d(f,g))=d(pr\circ f,pr\circ g)\in C^n(X;\pi_n(Y/B))$. However, $d(pr\circ f,pr\circ g)=0$ since $pr\circ f$ and $pr\circ g$ are assumed to be homotopic relative $A$, which contradicts our assumption of $f$ and $g$ not being homotopic relative $A$.
