It should be clear, first of all, that performing Hajj is only obligatory on those who can afford for it. If one cannot afford for Hajj and can only afford for `Umrah, it is absolutely permissible to perform `Umrah without Hajj. However, Hajj becomes obligatory on one when he can later afford for it.
Dr. Rif`at Fawzi, former professor of Shari`ah at Cairo Univ. , states: “If one can only afford for ‘umrah without hajj, there is nothing wrong with only performing `Umrah because there is no connection between Hajj and `Umrah.
If, afterwards, one can afford for Hajj, then one has to perform it; but if they could not, then Hajj is not obligatory on them.”
Allah Almighty knows best.