。
The question is whether he'll agree with us.
Literal
Problem [topic-は] he [subj-が] us [to-に] agree [whether-かどうか] [literary-である].
Another 問題は~かどうかである variant. 同意する ('agree, consent') takes its object/target with に, not と as in some other 'with' meanings. Note the に marking of 'us' — 'agree with us' in Japanese structures as 'to us, (he) agrees.'