This page has two good prayers to use when blessing gifts and tithes to God in church services, and prayers for offering oneself to God in worship. There is also a short article which looks at what the characteristics of Christian giving are:-
An Offertory Prayer
Lord, we thank for Your faithfulness.
Thank you that we can always trust in You.
You are an abundant God and out of Your great mercy you have given us so much.
We give you this offering today.
With it we worship You and give our whole selves to You.
Please now take it and use it for Your kingdom and Your glory.
Extend and multiple its reach and influence we pray.
May it be a great blessing to many.
We ask all this in the powerful name of Jesus.
(a short offering prayer suitable to use as a prayer for blessing the gifts of money during a service, from www.lords-prayer-words.com)