There is nothing absolutely definitive in the IRS code which spells out exactly what expenses you may deduct for business purposes. However, the IRS is very clear about the fact that in order to be deducted as a business expense, the expense must be something that is "ordinary and necessary" in your line of work. While a few hundred dollars for sponoring a softball team and buying their shirts may be allowed as a ordinary business expense, I would agree 100% with your accountant that deducting $10,000 for travel expenses for the team would not be considered ordinary or necessary.
It is one thing to pay for shirts that advertise your law firm's name, but to pay $10,000 for the team's travel expenses is not related to any advertising expense.
You can certainly claim the deduction, but if you were to be audited I would say with almost 100% certainty that the IRS would disallow this deduction.
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Thank you Chas