There is no need to use symbolic tools to solve problems via fzero, merely understanding how to use functions. However, your question is confusing.
I think you are saying, you want to perform a minimization that uses existing variables s and t. (I won't use o as a variable name. That is a terrible programming pattern to employ, as it will lead to bugs, when you mistake the variable o with the number 0.) Then, I'll replace element 5 of the variable t with an unknown. Again, ABSOLUTELY no need to use symbolic tools.
One little helper function, that will on the fly replace a specific element or elements of a vector or array.
function vec = replace_elem(vec,newelem,replaceind)
vec(replaceind) = newelem;
I could probably have done that using a function handle, but an m-file is easy too.
s = rand(1,30);
t = rand(1,30);
replind = 5;
fun=@(tnew) sum(replace_elem(t,tnew,replind) - s);
[tnew,fv,exitflag] = fzero(fun,1)
The function I created was a trivial one. But you were asking a generic question.