You can pass any information that needs to be propagated during authentication flow in a Context. Add desired redirect URL in a Context when you request identity providers from ACS.
For example:
https://YourNamespace.accesscontrol.windows.net/v2/metadata/IdentityProviders.js?protocol=wsfederation&realm=YourAppRealm&reply_to=YourAppReturnURL&context=YourRedirectionURL&version=1.0&callback=OptionalFunctionName
See this and this link for more info.
What is left is to get URL from Context when ACS post Token to your back-end. For example (needs code review):
var formResult = Request.Content.ReadAsFormDataAsync().Result;
var wresult = formResult["wresult"];
var signInResponse = new SignInResponseMessage(Request.RequestUri, wresult);
var rstr = new WSFederationSerializer().CreateResponse(signInResponse,new WSTrustSerializationContext(SecurityTokenHandlerCollectionManager.CreateDefaultSecurityTokenHandlerCollectionManager()));
var redirectURL = rstr.Context;
Regards, Matej