Find total spent on purchases, then the donation, then the original amount.

\$\begin{aligned}
\text{Spent on sandwich + juice} &= 15\% + 25\% = 40\%
\\
\text{Remaining after purchases} &= 60\%
\\
\text{Donation: 40\% of remainder} &= 0.40 \times 60\% = 24\% \text{ of original}
\\
\text{Total used} &= 40\% + 24\% = 64\%
\\
\text{Left} &= 100\% - 64\% = 36\% = \$36
\\
\text{Original} &= \$36 \div 0.36 = \$100
\end{aligned}\$

Final Answer: \$100