Chapter 48 states Sai is 'subservient to his devotees' but also that love is necessary for devotion. Isn't this a contradiction? Is Sai's vow of subservience conditional on the devotee's love?

There is no contradiction, but rather a description of the two sides of the divine relationship. According to Chapter 48, Sai's nature is to be a benefactor, and he acts as one who is 'subservient to his devotees' as a vow to show his pure love for them. This is his inherent disposition as the 'Uplifter of the poor' and the 'one full of love for those who seek refuge'. However, for the devotee to experience the fruits of this divine grace, their own inner state is crucial. As Chapter 48 also explains, devotion and knowledge are 'fruitless' and 'dry' without love, and love itself arises from faith. Therefore, Sai's vow is an expression of his unconditional grace, but the devotee's ability to receive and benefit from that grace is unlocked through their own faith and love.