Is our retention of donated funds for an indefinite period until the most optimal da'wah or charitable work arises considered a delay of funds for the poor and orphans and a sin upon us, or are we, by doing so, among those to whom Allah's saying applies: "And they withhold [even] the small necessities"? And must we inform donors that their donations will not be spent immediately and will remain with us for a period?

What you are doing is a righteous act for which you will be rewarded. You are considered agents on behalf of the donors, and you act according to what you have been duly authorized to do, whether by Islamic law or by the mandator. If the funds are Zakat, it is not permissible to withhold them from the poor and needy except to the extent necessary for their delivery. If they are Sadaqat (charity) and donations other than Zakat, they should be spent according to the purpose for which they were collected, and disbursed according to what is in the best interest, even if that entails a delay, unless the donor stipulated otherwise. This is because custom permits authorizing the charitable organization to distribute donations according to its system. However, if the donor stipulates a specific condition, it must be adhered to. There is no need to inform the donor that their donation may be delayed until a deserving recipient is found or until the benefit is realized.